Radiation-sensitive emulsions employed in photography are comprised of a dispersing medium, typically gelatin, containing radiation-sensitive microcrystals--known as grains--of silver halide. The radiation-sensitive silver halide grains employed in photographic emulsions are typically comprised of silver chloride, silver bromide, or silver in combination with both chloride and bromide ions, each often incorporating minor amounts of iodide. Iodide is typically present in concentrations of below about 10 mole percent, but can be present in concentrations as high as about 40 mole percent without creating a separate silver iodide phase, depending upon the temperature of grain formation. Silver halide grains of these compositions have isomorphic face centered cubic rock salt type crystal structures, and this is independent of the crystal faces the grains happen to be bounded by--e.g., {100} crystal faces, as is typical of cubic grains, {111} crystal faces, as is typical of octahedral grains, or some combination of these crystal faces.
Though infrequently employed in photographic applications, silver iodide emulsions are known. The most commonly encountered form of silver iodide crystals is the hexagonal wurtzite type, designated .beta. phase silver iodide. Silver iodide is also stable at room temperature in a face centered cubic zinc blende type crystalline form, designated .gamma. phase silver iodide.
Maskasky U.S. Pat. Nos. 4,094,684 and 4,142,900 and Koitabashi et al U.K. Patent Application No. 2,053,499A teach the use of silver iodide grains as host grains for the epitaxial deposition of silver chloride and silver bromide. Such emulsions advantageously combine the light absorbing capabilities of silver iodide with the latent image forming and processing characteristics of silver chloride and silver bromide to produce useful radiation-sensitive photographic emulsions.
Koitabashi et al European Patent Application No. 0019917 (published Dec. 10, 1980) discloses epitaxially depositing on silver bromoiodide grains containing from 15 to 40 mole percent iodide, silver halide which contains less than 10 mole percent iodide. The unusually high iodide levels in the host grains are necessary to prevent the indiscriminate deposition of the silver halide on the surfaces of the host grains. From the composition of both the host grains and the silver halide deposited thereon, it is apparent that both are comprised of face centered cubic rock salt type crystal structures.
The indiscriminate deposition of silver salts on face centered cubic rock salt type crystal structure silver halide grains has been suggested from time to time in the art. For example, Berriman U.S. Pat. No. 3,367,778 suggests the use of a variety of silver salts to form the core and/or shell of surface fogged core-shell grains. The silver salts are preferably silver halides, but additionally include silver thiocyanate, silver phosphate, silver cyanide, and silver carbonate.
Walters et al U.S. Pat. No. 3,782,960 discloses direct-print silver halide emulsions which can be light developed or processed by conventional developing-out techniques. It is claimed that increased sensitivity and background stability under latensification are achieved by sensitizing silver halide grains, such as converted-halide silver chlorobromide grains, with 0.01 to about 25 mole percent iodide, from about 0.001 to about 1.0 mole percent gold, and an effective quantity of silver thiocyanate. From electron micrographs of emulsion samples prepared according to Walters no evidence has been found of silver thiocyanate being epitaxially located on the silver halide grains or of the silver thiocyanate being confined to selected sites of the grains.